YY1 and breast carcinoma: Further reinforcing the previous findings, YY1 was shown to hinder the growth of human breast carcinoma and glioblastoma cells by inhibiting proliferating cell nuclear antigen (PCNA) expression, a marker of cell growth, and reducing phosphorylation of retinoblastoma protein (pRbSer249/Thr252), which is critical for cell cycle progression [45].